[RFC PATCH v2 3/7] RISC-V: Prefer sstc extension if available

From: Atish Patra
Date: Fri Mar 04 2022 - 15:15:01 EST


RISC-V ISA has sstc extension which allows updating the next clock event
via a CSR (stimecmp) instead of an SBI call. This should happen dynamically
if sstc extension is available. Otherwise, it will fallback to SBI call
to maintain backward compatibility.

Signed-off-by: Atish Patra <atishp@xxxxxxxxxxxx>
---
drivers/clocksource/timer-riscv.c | 21 ++++++++++++++++++++-
1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/drivers/clocksource/timer-riscv.c b/drivers/clocksource/timer-riscv.c
index 1767f8bf2013..d9398ae84a20 100644
--- a/drivers/clocksource/timer-riscv.c
+++ b/drivers/clocksource/timer-riscv.c
@@ -23,11 +23,24 @@
#include <asm/sbi.h>
#include <asm/timex.h>

+static DEFINE_STATIC_KEY_FALSE(riscv_sstc_available);
+
static int riscv_clock_next_event(unsigned long delta,
struct clock_event_device *ce)
{
+ uint64_t next_tval = get_cycles64() + delta;
+
csr_set(CSR_IE, IE_TIE);
- sbi_set_timer(get_cycles64() + delta);
+ if (static_branch_likely(&riscv_sstc_available)) {
+#if __riscv_xlen == 32
+ csr_write(CSR_STIMECMP, next_tval & 0xFFFFFFFF);
+ csr_write(CSR_STIMECMPH, next_tval >> 32);
+#else
+ csr_write(CSR_STIMECMP, next_tval);
+#endif
+ } else
+ sbi_set_timer(next_tval);
+
return 0;
}

@@ -165,6 +178,12 @@ static int __init riscv_timer_init_dt(struct device_node *n)
if (error)
pr_err("cpu hp setup state failed for RISCV timer [%d]\n",
error);
+
+ if (riscv_isa_extension_available(NULL, SSTC)) {
+ pr_info("Timer interrupt in S-mode is available via sstc extension\n");
+ static_branch_enable(&riscv_sstc_available);
+ }
+
return error;
}

--
2.30.2